Table 1.

Baseline characteristics of the study population (N = 41,102)

Participant characteristic
Age (years), mean (SD)55.0 (9.8)
BMI (kg/m2), mean (SD)25.4 (4.2)
Age at menarche (years), mean (SD)13.1 (1.5)
 Missing1.9 (793)
Parity, % (N)
 012.1 (4,979)
 114.1 (5,792)
 248.4 (19,901)
 ≥325.2 (10,363)
 Missing0.2 (67)
Age at first birth (years), mean (SD)27.1 (5.2)
 Missing, % (N)5.6 (2008)
Menopausal status, % (N)
 Premenopausal40.2 (16,506)
 Perimenopausal/unknown3.3 (1,349)
 Postmenopausal56.5 (23,247)
Ever OC use, % (N)79.3 (32,602)
 Missing0.5 (201)
HRT use, % (N)
 Never74.9 (30,791)
 Former14.5 (5,945)
 Current3.7 (1,521)
 Missing6.9 (2,845)
Benign breast disease, % (N)21.7 (8,919)
 Missing2.0 (811)
Family history breast cancer, % (N)12.7 (5,236)
 Missing3.6 (1,471)
Total breast volume (cm3), median (IQR)760 (493–1,106)
Absolute dense volume (cm3), median (IQR)57 (42–77)
Percent dense volume (%), median (IQR)7.7 (5.3–11.7)